Direct Observation of Long Exciton Diffusion Lengths in C70 Layers

Abstract: Organic solar cells hold great promise of becoming an inexpensive, large-area, flexible and environmentally-friendly technology. They rely on the ability of fragile Frenkel excitons – quasiparticles that result from light absorption – to migrate to the interface between two organic semiconducting materials (the donor and the acceptor). Short Frenkel exciton diffusion lengths (<10 nm) in typical organics necessitate the use of the so-called bulk heterojunctions (BHJs), a nanotextured interpenetrating network between the donor and acceptor materials. The BHJ presents a compelling compromise between two contradictory requirements: short exciton diffusion to the interface and long charge migration to the electrodes. The BHJ is notoriously difficult to control and optimize due to its spontaneous formation and self-organization. However, the very need of BHJs may be fully waived if exciton diffusion lengths increase sufficiently to match light penetration depths. Furthermore, a number of current concepts in organic optoelectronics might be reconsidered if the exciton diffusion lengths would match the characteristic spatial scale of the devices.
Here we directly demonstrate surprisingly large exciton diffusion distances (>50 nm) in C70 fullerene layers. For this, we developed a unique time-of-flight approach to follow the exciton dynamics by ultrafast photoinduced absorption spectroscopy. Efficient exciton harvesting of >50% even from a 48 nm C70 layer clearly points towards extremely high exciton diffusion rate. All these suggest high potential of development of simple layered organic solar cells with high light harvesting efficiency also applicable to organic thin-film transistors and light-emitting transistors.Bio: Dr. Maxim S. Pshenichnikov is currently a professor at the Zernike Institute for Advanced Materials at the University of Groningen. He received his PhD in physics from Moscow State University in 1987. His research interests are ultrafast processes wherever he finds them to occur: in molecules, liquids, plastics, or at interfaces.